Previous studies have shown that the effect of emotional distractor on working memory (WM) is mixed. In neuroimaging studies, the activity of fronto-parietal cortex related to WM is reduced when emotional distractor presented. In EEG studies, the parietal P3 component is related to WM process. However, the acute stress effect on WM related P3 component is still unclear. To this end, we conducted an EEG study by implementing an working memory paradigm to examine how the acute stress affects ongoing working memory processing. In this paradigm, acute stress/neutral movie clips were used to induce stress status. Participants performed an N-back task with sequentially presented number, in which they were asked to identify whether the current number is identical with the one presented two trials before (i.e., 2-back, higher WM load) or with 1 (0-back, lower WM load). The heart beats and subjective feeling were recorded for emotional mood measurement. The heart beats and negative emotional feeling were significantly higher in the stress group compare to neutral group, which suggests successfully acute stress induction. The amplitude of parietal P3 was significantly bigger in the 0-back condition than in the 2-back condition. These results were consistent with previous EEG studies using N-back task, which have shown that the smaller P3 in high WM load condition. Furthermore, compare to the neutral condition, the acute stress has stronger effect on P3 in 0-back task than in 2-back task. These results can easily explained by the recently proposed dual competition model positing that emotional and cognitive processes, when operating simultaneously, compete with each other for limited neural resources. Specifically, processing of emotional stimuli may interfere with ongoing cognitive processes when cognitive resources are not entirely devoted to goal-directed processing under a lower task demanding condition, such as during the 0-back task in our study. When more resources are required to achieve goal-directed behavior such as in the 2-back WM task here, however, the interference effect is attenuated due to the suppression of stress processing. Together, these results suggest that P3 is a reliable marker for WM load in n-back task. Furthermore, these results provide direct evidence for the dual competition model.